If you’ve ever looked at a bag of clay or a glaze recipe and seen “cone 6” or “cone 10” without fully understanding what it means, this guide is for you. The cone system is one of the most important concepts in ceramics — it determines whether your clay vitrifies, your glaze melts correctly, and your finished piece is food safe and durable.
What is a Pyrometric Cone?
A pyrometric cone is a small, elongated pyramid made from a precisely controlled mixture of ceramic materials. When placed inside a kiln, it bends and melts at a specific temperature — giving you a reliable visual indicator of the heat work achieved during firing.
The key word here is heat work — not just temperature. Heat work accounts for both temperature and time. A kiln that reaches 2300°F quickly does less heat work than one that reaches 2300°F slowly. This is why cones are a more accurate measure of a firing than temperature alone.
How the Cone Numbering System Works
Cone numbers run from 022 (the lowest, coolest) up through 0 and then 1 through 14 (the hottest). The numbering can be confusing at first because the scale goes from high “0x” numbers (low temperature) down through single digits and then back up:
- Low fire: Cone 022 – Cone 1 (approximately 1087–2109°F / 586–1154°C)
- Mid fire: Cone 2 – Cone 7 (approximately 2124–2295°F / 1162–1257°C)
- High fire: Cone 8 – Cone 14 (approximately 2305–2489°F / 1263–1365°C)
The most commonly used firing ranges in studio ceramics are cone 06/04 (low fire bisque and earthenware), cone 6 (mid fire, the most popular for home and studio electric kilns), and cone 10 (high fire, traditional for gas reduction kilns).
Cone Temperature Reference Chart
The temperature at which a cone bends depends on how fast the kiln is firing. The Orton cone chart lists temperatures for different firing rates — the most commonly referenced is the 108°F/hour (60°C/hour) column, which represents a slow final climb to peak temperature.
| Cone | Temperature (°F) — 108°F/hr | Temperature (°C) | Common Use |
|---|---|---|---|
| 022 | 1087°F | 586°C | Luster glazes, overglaze enamels |
| 06 | 1828°F | 998°C | Low-fire bisque, earthenware |
| 04 | 1945°F | 1063°C | Standard bisque, earthenware glaze |
| 02 | 2048°F | 1120°C | Upper range earthenware |
| 1 | 2109°F | 1154°C | Upper low fire |
| 6 | 2232°F | 1222°C | Mid-fire standard — most electric kilns |
| 10 | 2345°F | 1285°C | High fire — gas reduction, porcelain |
| 12 | 2383°F | 1306°C | Very high fire |
Witness Cones vs Kiln Sitter Cones
Witness Cones
Witness cones are small pyrometric cones placed inside the kiln on a cone pack — a small slab of clay with three cones embedded in it. You use three cones at once: one cone below your target, one at your target, and one above. For a cone 6 firing:
- Cone 5 — guard cone (should be fully bent)
- Cone 6 — target cone (should be bent to 90°)
- Cone 7 — guard cone (should not be bent)
After firing, you check the cone pack to verify the kiln actually reached the right temperature. If cone 6 is only partially bent, you underfired. If cone 7 is fully bent, you overfired. Witness cones are the most reliable way to verify your kiln’s thermocouple is accurate — thermocouples drift over time and can read incorrectly, especially in older kilns.
Kiln Sitter Cones
Kiln sitter cones are used in older kilns with a kiln sitter mechanism — a device that shuts the kiln off automatically when the cone bends. These are small bars rather than the elongated cone shape. If your kiln has a kiln sitter, always use witness cones alongside it to verify the shutoff is triggering at the right point.
Firing Rate and How It Affects Cones
The same cone number bends at different temperatures depending on how fast the kiln is firing. A fast firing (270°F/hour final rate) needs a higher temperature to achieve the same heat work as a slow firing (108°F/hour). This is why the Orton cone chart has multiple columns — and why firing too quickly can result in underfired work even if the thermocouple says you hit the right temperature.
As a general rule: slow and steady firing produces more consistent results, especially for glazes that need time to develop fully.
Choosing the Right Cone for Your Work
- Cone 04 — standard bisque for all clay bodies. Most clay is bisque fired to cone 04 regardless of its final glaze firing temperature.
- Cone 6 — the most practical range for home studio electric kilns. Wide range of commercial and studio glazes available. Most mid-fire clay bodies vitrify well here.
- Cone 10 — traditional for gas reduction kilns and high-fire stoneware and porcelain. Produces more subtle, atmospheric glaze effects. Harder on kiln elements in electric kilns.
For more on how cone temperature relates to clay maturity, see our guide on What is Vitrification. For high-fire clay body options, see High Fire Clay Bodies.
